Diffusion-limited and asymmetric growth of amorphous layer in Ni/Zr bilayer upon annealing
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Diffusion-limited and asymmetric growth of amorphous layer in NiyyZr bilayer upon annealing W. S. Lai and B. X. Liua) Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China, and State Key Laboratory of Solid-State Microstructure Physics, Nanjing University, Nanjing 210093, China (Received 14 March 1997; accepted 20 August 1997)
Asymmetric growth of amorphous layer in a NiyZr bilayer, in which a thin disordered interlayer is preset, upon annealing at medium temperatures is observed by molecular-dynamics simulation with an n-body potential. It is shown that the amorphous layer is extended from the interlayer with different speeds toward two opposite directions and that the growth kinetics follows time dependence of t 1/2 , indicating amorphization upon annealing in the NiyZr bilayer is indeed through a diffusion-limited reaction. Besides, two low temperature limits allowing the growth of amorphous layer toward Ni and Zr layers are also obtained.
I. INTRODUCTION
As atomistic simulations are widely recognized as a novel means for a better understanding of microscopic processes, the molecular-dynamics (MD) simulation has been employed to investigate the mechanism for glass transition in binary metal systems.1–3 For instance, Massobrio et al. reported that a crystalline NiZr2 underwent crystal-to-amorphous (C-A) transition by irradiation-induced chemical disorder and claimed that chemical disorder was a major driving force behind the C-A transition. In their simulation, chemical disorder was induced by random exchanging Ni and Zr atoms,2 and no long-range-diffusion was observed during the whole process. Weissmann et al. simulated Zr –Co superlattice with sharp interface upon annealing and claimed that all the Co atomic planes became disordered at a temperature as high as 827 ±C,3 yet they did not observe diffusion below this temperature either. Experimentally, however, many solid-state reaction studies confirmed the formed amorphous phase was definitely an alloy and therefore an alloying process through long-range mutual diffusion of two constituent metals was necessary.4–8 One may conjecture that chemical disorder is also a major driving force responsible for solid-state amorphization like that in irradiation-induced amorphization simulated by Massobrio et al.,2 to which further evidence is still required. For the Ni–Zr system, it is well known that amorphous alloy can be produced by various techniques, such as liquid melt quenching, ion irradiation, solidstate reaction, etc.9–11,5–8 In the meantime, many MD simulations have been performed to study amorphization a)
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J. Mater. Res., Vol. 13, No. 6, Jun 1998
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in the Ni–Zr system upon liquid melt quenching,12,13 ion irradiation,2,14 and applying load.15 In this regard, to the authors’ knowledge, no MD study has been hitherto reported concerning an atomis
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